A solar photovoltaic (PV) array is a parallel arrangement of strings of series-connected PV panels. A string in the context of PV arrays, often referred to simply as a “PV string,” is an arrangement of PV panels connected together in series to produce a voltage at a desired level for a downstream DC-to-AC inverter. As will be appreciated, solar PV arrays can occupy extensive surface areas. A typical PV system might contain multiple PV arrays and dozens or even hundreds of strings of PV panels.
A PV array is composed of multiple strings of PV panels. Each string output is received in a device called a combiner, which “combines” the currents from multiple strings into a larger conductor, called a busbar, for feeding downstream equipment and ultimately to a DC-to-AC inverter that converts the DC currents produced by the strings of PV panels into an AC current. The combiner can receive strings from multiple PV arrays.
In the combiner, which typically takes the form of an enclosure referred to as a “string combiner box,” protection devices such as circuit breakers or fuses are used to protect against backfeeding current from one or more strings into another string. Backfeeding current into a PV panel can damage the wiring panel or otherwise adversely affect the performance of the PV array.
Both grounded and ungrounded PV systems are in widespread use. Even in the United States, where grounded electrical power distribution systems have historically been required, ungrounded PV systems are now permitted, provided a DC disconnect is provided for ungrounded conductors.
Typically, in a disconnect device, up to four poles are present, with each pole having a finite rated interrupting capacity at a certain maximum voltage per pole (for example, 250V per pole). This means that if all four poles are utilized in a PV system, the maximum system voltage that can be supported by a single protection device is 1000V. But if the user desires to ground one of the polarities (typically negative), there is no pole inside the disconnect device available to be disconnected simultaneously with the poles of the other polarity (typically positive). Thus, the user is faced with a Hobson's choice of leaving the grounded polarity in a “live” state where it cannot be disconnected, or reducing the system voltage supported to 750V to free up one of the poles for connection to the grounded polarity. In other words, until now, the user has to choose between total isolation of the source (better safety) and system voltage capacity. The present disclosure allows the user to have the proverbial cake and eat it, too.
FIGS. 1A-1D illustrate the limitations of existing configurations. In FIG. 1A, a 750V application (assuming 250V/pole) using a three-pole (3P) protection device is shown in a Grounded configuration 100. In this Grounded configuration, the positive polarity is disconnected using a 3P disconnect device, but the negative polarity is not. This configuration 100 uses a less expensive 3P device but does not allow disconnect of both polarities. To disconnect both polarities, a 4P device would have to be used as shown in FIG. 1B.
In FIG. 1C, a 1000V application using a four-pole (4P) protection device is shown in an ungrounded configuration 110. Here, three of the four poles are used to disconnect the positive polarity, and the remaining fourth pole is used to disconnect the negative polarity, allowing total disconnect of both polarities on a 1000V system. However, it is not possible to ground the negative polarity.
If the user wants to ground the negative polarity in a 1000V system, the user must leave the negative polarity incapable of being disconnected, as shown in FIG. 1D. In FIG. 1D, a 1000V application using a four-pole (4P) protection device is shown in a grounded configuration. But all four poles are used to disconnect the positive polarity, requiring the negative polarity to be incapable of being disconnected. Thus, only one of the polarities can be disconnected in this configuration. As can be seen between FIGS. 1A through 1D, the user has the choice to either disconnect only one of the two polarities or to leave one of the polarities ungrounded. This is not acceptable for users who desire to ground one of the polarities and to disconnect both polarities using a four-pole disconnect for a 1000V application (assuming 250V/pole).